The subject matter of the present invention relates to a seismic data interpretation method and apparatus, and, more particularly, to a method and apparatus for operating on pairs of data obtained from either a repeat seismic operation or for operating on pairs of attributes derived from a single seismic operation performed on the earth's surface where the pairs of data are representative of characteristics of the earth's formation and generating an output record medium from which the characteristics of the earth formation may be determined.
Seismic data is obtained by measuring and recording the data during a "3D seismic operation" performed using a set of geophones situated on the surface of the earth.
A "seismic operation" in general is performed by detonating an explosive energy source at the surface of the earth and, using a set of geophones, measuring a set of sound wave vibrations emerging at the earth's surface but displaced by a plurality of distances (x, x+50 feet, x+100 feet, etc) from the location of the explosive energy source. This set of emerging sound vibrations, produced during the "seismic operation", may be called "2D" seismic data because, in our example, the sound vibrations are propagating in the x-z plane.
However, in reality, the sound vibrations are propagating in three dimensions, that is, the vibrations are propagating along the x, y, and z axes. As a result, the emerging seismic data, emerging from the earth's surface near the set of geophones, should more properly be called "3D" seismic data.
During a seismic operation, the sound vibrations from the explosive energy source will propagate downwardly into the earth, reflect off various earth layers (such as rock layers or sand shale), and propagate upwardly to the earth surface. As a result, the strength of each sound vibration reflecting off an earth layer is a function of the impedence of the earth layer within the earth. Consequently, the 3D seismic data recorded by the geophones at the earth's surface represents a set of characteristics of the earth's formation located within the earth and situated below the set of geophones.
Recall that the aforementioned "3D" seismic data is generated and recorded by the geophones when the 3D seismic operation is performed at the earth's surface, the 3D seismic data representing a set of sound vibrations propagating in three dimensions within the earth (that is, along the x, y, and z axes). However, there is a fourth dimension that should be considered as well which would result in the generation and recordation, by the set of geophones, of "4D" seismic data. That fourth dimension is called "time", or "t". As a result, the four dimensions would be (x, y, z, and t), where x is the horizontal distance, y is the transverse distance, z is the reflection time, and t is the elapsed time.
The 4D seismic data is generated and recorded by the geophones when a "repeat seismic operation" is performed at the surface of the earth. A "repeat seismic operation" is performed in the following manner: performing a 3D seismic operation at a location of the earth's surface at time "t1", and then, at a later time "t2", performing another 3D seismic operation at the same location of the earth's surface at time "t2". If the earth layers, located within the earth below the set of geophones, have changed in any manner (e.g., the new presence of oil, gas, or fluid content), the 3D seismic data emerging from the earth at time "t2" will be different from the 3D seismic data which emerged from the earth at time "t1".
As a result of the performance of the "repeat seismic" operation, two (2) sets of seismic data are measured by the set of geophones: a first set of the seismic data measured by the geophones during time "t1", and a second set of the seismic data measured by the geophones during time "t2".
There is an alternate method practiced for generating the two sets of seismic data. During a single 3D seismic operation, a single set of seismic data is measured by the set of geophones and derived from the 3D seismic operation. However, that single set of seismic data is later processed in two different ways. As a result of the two different ways of processing the single set of seismic data, a pair of attributes are obtained, that is, the two (2) sets of seismic data comprising a first set of seismic data and a second set of seismic data are obtained. This alternate method of generating the two sets of seismic data is hereinafter called the "single seismic two processing method operation".
In addition, multiple data sets can be recorded at the same time, but measuring different types of wave propagation, such as compressional and shear waves. These can be considered a single data set processed two ways.
However, when the first and second sets of seismic data, representative of characteristics of an earth formation, are measured and recorded or otherwise obtained, a processing system located within a workstation should analyze or interpret the first and second sets of seismic data to determine the characteristics of the earth formation situated below the set of geophones at the earth's surface. The processing system must include a special software package (hereinafter called "Cubemath software") which is specifically designed to analyze or interpret the first and second sets of seismic data that were obtained from either the "repeat seismic operation" or the "single seismic two processing method operation". However, heretofore, there existed no such special software package.
As a result, a need exists for a special software package (known as the "Cubemath software") which, when stored in a workstation processing system, will allow the processing system to analyze and interpret the first and second sets of seismic data obtained from either the repeat seismic operation or the single seismic two processing method operation for the purpose of determining the characteristics of the earth formation located below a set of geophones situated at the earth's surface.